Height of the airplane above the ground = 38,900 ft.
Angle of depression from the plane to the base of the building = 4.3°.
So, we could apply tangent trigonometric ratio:

tan 4.3° =

0.07519 =

On cross multiply, we get
0.07519 x= 38,900.
Dividing both sides by 0.07519, we get
x= 517356.03 ft.
Therefore, the plane must fly 517356.03 ft. horizontally to be directly over the building.